Electronic circuit, and method of operating a high-pressure lamp

ABSTRACT

The invention relates to an electronic circuit and to a method of operating a high-pressure lamp in various operational modes. The circuit comprises a DC-AC converter consisting of two bridges ( 110 - 1 ) and ( 110 - 2 ). A series circuit comprising a first coil L 1 , the high-pressure lamp ( 120 ), and a second coil L 2  is connected between the outputs ( 112 - 1 ) and ( 112 - 2 ) of these two half bridges. The invention has for its obeject to develop such a known circuit further such that it is equally suitable for an ignition mode and for a normal operational mode of the high-pressure lamp, without individual components having to be overdimensioned. According to the invention, this object is achieved in that a first capacitor C 1  is connected in a path from the junction point of the first coil L 1  and the high-pressure lamp ( 120 ) either to a reference potential (−) or to an operating potential (+), and in that a second capacitor C 2  is connected in the path from the junction point of the high-pressure lamp ( 120 ) and the second coil L 2  either to the reference potential (−) or to the operating potential (+) or in parallel to the high-pressure lamp ( 120 ).

The invention relates to an electronic circuit and to a method ofoperating a high-pressure lamp in an ignition mode and in a normaloperational mode.

Such circuits are known from the prior art, for example from U.S. Pat.No. 4,734,624. The circuit known from the cited US document is shown inFIG. 7 and will be explained in detail below. It comprises a DC-ACconverter which comprises the four transistors I1, I2, I3, and I4, suchthat the transistors I2 and I3, as well as I1 and I4 are connected inseries, respectively, thus forming a half bridge each time. The two halfbridges are connected in parallel between an operating potential (+) anda reference potential (−). A freewheel diode 21 to 24 is connected inparallel to each of the individual transistors I1 to I4. The halfbridges act as a DC-AC converter and provide a suitable AC current forthe operation of the high-pressure lamp in the ignition mode or in thenormal operational mode. The high-pressure lamp itself forms part of aseries circuit comprising a first coil 1, followed by the high-pressurelamp L, and followed again by a second coil 3. This series circuit isconnected between the outputs S and T of the two half bridges. Theseries circuit is completed with a capacitor 2 which is connected inparallel to the high-pressure lamp L and the second coil 3.

As long as the high-pressure lamp has not been ignited, it represents aninterruption of said series circuit. This interruption, however, isbridged by the capacitor 2, so that the two half bridges areinterconnected by the capacitor 2. An independent operation of the twohalf bridges is accordingly not possible, also in the non-ignited stateof the high-pressure lamp L.

The first coil 1 and the capacitor 2 are to be dimensioned for thenormal operational mode such that they act as a filter for filtering outthe AC component from the lamp current. They are definitely not operatedin a resonant mode during this, i.e. the switching frequencies of thetwo transistors I2 and I3 are substantially higher than the resonancefrequency of a resonant circuit formed by the first coil 1 and thecapacitor 2.

The first coil 1 and the capacitor 2 are to be operated at theirresonance frequency in the ignition mode by contrast, so as to generatea high voltage required for igniting the high-pressure lamp L. It isnecessary for this that the capacitor 2 is constructed so as to beresistant to high voltages, i.e. for a few kV. In addition, the firstcoil 1 should be dimensioned such that it does not enter the saturatedstate even when loaded by an ignition current, which is approximatelyten times stronger than the current during normal operation.

The prior art embodiments show that a completely different dimensioningof the components, in particular of the first coil 1 and the capacitor2, is necessary for the ignition mode as compared with the normaloperational mode. The circuit known from the prior art and shown in FIG.7, therefore, can be usefully employed basically either for an ignitionmode or for a normal operational mode. It is indeed possible to use thecircuit of FIG. 7 for a combination of ignition mode and normaloperational mode, but in that case the first coil 1 and the capacitor 2will be strongly overdimensioned for the normal operational mode if itis to be capable of realizing the ignition mode as well.

A further disadvantage of the circuit of FIG. 7 is that stronghigh-frequency synchronous interference voltages, which arise duringswitching of the switching elements I1 . . . I4, can be transmittedunchecked to the connection lines 121, 122 of the high-pressure lamp Lbecause the two half bridges are not decoupled, but are coupled via thecapacitor 2. Losses will eventually occur during switching of thetransistors I1 . . . I4 which are higher in proportion as the switchingfrequencies are higher. No measures are taken in the circuit of FIG. 7for reducing these losses.

Starting from the cited US patent, it is accordingly an object of theinvention to develop the circuit for supplying high-pressure lampsdisclosed therein further such that it renders possible both an ignitionmode and a normal operational mode, i.e. a stable continuous operation,of a high-pressure lamp without the components of the circuit having tobe overdimensioned.

This object is achieved by the characterizing features of claim 1. Moreaccurately, the object of the invention is achieved in that thecapacitor 2 known from FIG. 7, denoted first capacitor hereinafter, isswitched from the junction point between the first coil and thehigh-pressure lamp either to the reference potential or to the operatingpotential, and in that a second capacitor is provided, which is switchedfrom the junction point between the high-pressure lamp and the secondcoil either to the reference potential or to the operating potential orin parallel to the high-pressure lamp.

The arrangement of the capacitors in accordance with the inventionachieves that the two half bridges are operated in a decoupled manner inprinciple, i.e. at least as long as the high-pressure lamp is notignited. This is also true for the case in which the second capacitor isconnected in parallel to the high-pressure lamp, because according tothe invention at least the high-frequency components can be drained offvia the common junction point—i.e. between the first coil and thehigh-pressure lamp—and the first capacitor also in this case.

The decoupling of the two half bridges described above renders possiblean independent construction and dimensioning of both a first resonantcircuit, also denoted filter circuit, comprising the first coil and thefirst capacitor for filtering out high-frequency components from thelamp current, in particular in the operational mode, and of a secondresonant circuit comprising the second coil and the second capacitor forigniting the high-pressure lamp in the ignition mode. The second coilfulfills an additional filtering function in the operational mode inthat it filters out high-frequency components from the lamp current.

The circuit according to the invention thus renders it possible torealize a filtering function in the normal operational mode as well asan ignition function in an ignition mode by means of independent circuitelements, i.e. the filter circuit and the second resonant circuit. Thetwo resonant circuits are dimensioned and operated independently of oneanother. An overdimensioning of in particular the first coil and thefirst capacitor in the filter circuit is not necessary, according to theinvention, because the filter circuit does not have to realize also theignition function.

Furthermore, the two terminals of the high-pressure lamp according tothe invention are connected either to the operating potential or to thereference potential via the first and the second capacitor—at least forhigh-frequency components. The high-frequency interference peaks arisingin the half bridges, for example owing to switching of the switchingelements, are automatically and advantageously drained off through thecapacitors before reaching the high-pressure lamp in this manner. Theactual current through the high-pressure lamp itself is accordinglyadvantageously at least substantially free from HF interference peaks.The supply leads to the high-pressure lamp are also free from DCinterference voltages then.

To reduce the switching losses in the transistors of the half bridges,various embodiments for capacitor arrangements connected in parallel tothe transistors are proposed in the dependent claims.

Advantageously, the circuit according to the invention comprises acurrent control for adjusting the amplitude of the current through thehigh-pressure lamp.

The object of the invention mentioned above is furthermore achieved by amethod of operating a high-pressure lamp as claimed in claim 8. Theadvantages of this method correspond substantially to the advantagesmentioned above for the circuit. In addition to these advantages, theclaimed method offers the possibility of operating the second resonantcircuit L2, C2 not only at its natural resonant frequency, butalternatively also at an odd fraction of its resonant frequency, whichhas the advantage that the losses in the ignition mode are clearlyreduced, in particular in the switching elements of the second halfbridge.

Further advantageous embodiments of the circuit according to theinvention and of the method according to the invention for the operationthereof are the subject of the dependent claims.

The invention is illustrated in seven Figures, in which:

FIG. 1 shows a first embodiment of the circuit according to theinvention;

FIG. 2 shows the lamp voltage during resonant excitation in the ignitionmode;

FIG. 3 shows the lamp current in a run-up phase of the high-pressurelamp;

FIG. 4 shows the lamp current in a positive half wave in the normaloperational mode;

FIG. 5 shows a second embodiment of the circuit according to theinvention;

FIG. 6 shows a third embodiment of the claimed electronic circuit; and

FIG. 7 shows a circuit known from the prior art for operating ahigh-pressure lamp.

The hardware construction of a preferred embodiment of the claimedcircuit will be explained in detail below with reference to FIG. 1,followed by an explanation of its operation in various operational modeswith reference to FIGS. 2 to 4.

FIG. 1 shows a preferred embodiment of the electronic circuit 100according to the invention for operating a high-pressure lamp 120 invarious operational modes, in particular in an ignition mode, a run-upmode, and a normal operational mode.

The circuit 100 comprises a DC-AC converter consisting of a first halfbridge 110-1 and a second half bridge 110-2 for providing a suitablealternating current for the high-pressure lamp 120 in said operationalmodes. The first half bridge 110-1 consists of two switching elementsconnected in series, preferably power transistors T1, T2, a DC voltageV_(DC) being applied to this series arrangement. The DC voltage is givenby a potential difference between an operating potential (+) and areference potential (−). The second half bridge 110-2 is constructed soas to be symmetrical to the first half bridge 110-1. The secondhalf-bridge 110-2 comprises two switching elements connected in series,preferably power transistors T3 and T4, which second half bridge 110-2is connected to said DC voltage V_(DC) in parallel to the first halfbridge 110-1.

In addition to the two half bridges 110-1, 110-2, the circuit 100according to the invention comprises a series arrangement which connectsthe output 112-1 of the first half bridge 110-1 to the output 112-2 ofthe second half bridge 110-2. The series arrangement comprises a firstcoil L1, followed by the high-pressure lamp 120 connected via a firstsupply line 121, followed again by a second supply line 122 and a secondcoil L2. The connection terminal of the first coil L1 not connected tothe high-pressure lamp 120 is connected to the output 112-1 of the firsthalf bridge 110-1, and the connection terminal of the second coil L2 notconnected to the high-pressure lamp 120 is connected to the output 112-2of the second half bridge 110-2.

A first capacitor C1 is connected in the path from the junction point ofthe first coil L1 and the high-pressure lamp 120 either to the operatingpotential (+) (not shown in FIG. 1) or to the reference potential (−).In addition, a second capacitor C2 is connected in the path from thejunction point of the high-pressure lamp 120 and the second coil L2either to the operating potential (+) (not shown in FIG. 1) or to thereference potential (−) or in parallel to the high-pressure lamp 120(shown in FIG. 5).

The embodiment of the electronic circuit according to the inventionshown in FIG. 1 further comprises several components for achieving acontrol of the lamp current level. For this purpose, a sensor device 130is provided between the first coil L1 and the connection terminal of thefirst capacitor C1 for generating a current sensor signal whichrepresents the level of the current through the first coil L1. Thiscurrent sensor signal is supplied to a comparator device 140 whichcompares the level of the current through the first coil L1 representedby the current sensor signal with a given reference current value I_(R),generating control signals for achieving a suitable control of theswitching elements T1, T2 of the first half bridge 110-1 in dependenceon the result of this comparison. More in detail, said control signalsare arranged such that they vary the duty cycles of the individualswitching elements T1 and/or T2 of the first half bridge 110-1 such thatthe average value of the current through the first coil L1 is adjustedto the desired value of the lamp current. The duty cycle defines theratio of the switch-on time of a switching element to the cycleduration, for example of the current. The control of the current throughthe first coil L1 at the same time controls the instantaneous value ofthe lamp current through the high-pressure lamp 120 because of thearrangement of the circuit according to the invention.

In addition to the comparator device 140 described above, a delay device150 may be provided for delaying the control signals by a given delaytime with respect to the moment when it is detected that the levelbecomes too high or too low in comparison with the reference currentvalue I_(R). This delay time has a damping influence of the control. Thedelay time is preferably chosen such that at least a desired criticaldamping adjusts itself in the filter circuit comprising the second coilL2 and the first capacitor C1, so that any control deviation detected iscompensated without overshoot. At the same time, the delay time isadjusted such that the current through the first coil L1 changes itssign at least twice during one switching cycle.

The operation of the circuit described above and shown in FIG. 1 will beexplained below for various operational modes.

1. Ignition Mode In the non-ignited state, the high-pressure lamp 120 isto be regarded as an interruption, i.e. it disconnects the first halfbridge 110-1 with the filter circuit connected thereto, comprising thefirst coil 1 and the first capacitor C1, from the second half bridge110-2 with the second resonant circuit connected thereto, comprising thesecond coil L2 and the second capacitor C2. This decoupling renders itpossible to excite the second resonant circuit at its natural resonancefrequency f_(R2) so as to make available a sufficiently high ignitionvoltage for the high-pressure lamp 120. The excitation of the resonancenecessary for this in the second resonant circuit takes place such thatthe switching elements T3 and T4 of the second half bridge 110-2 areswitched on and off in alternation either at said resonance frequencyf_(R2) or an odd fraction thereof. The resonant resistance of the secondresonant circuit is suitably chosen such that, at the ignition voltageof, for example, 5 kV necessary for the high-pressure lamp 120, thecurrent in the second coil L2 is not higher than the maximum lampcurrent during a normal operational mode yet to be described below. Sucha construction will lead to the high resonance frequency mentionedabove.

If the desired ignition voltage is to be generated only throughexcitation of the second resonant circuit with an odd fraction, forexample ⅕or ⅓, of the resonance frequency f_(R2), then the minimumquality of the second resonant circuit must be made correspondinglyhigher. FIG. 2 shows the current and voltage gradients in the secondcoil L2 and the resulting ignition voltage for the high-pressure lamp120 in the case of resonant excitation of the ignition voltage by meansof the third harmonic.

Typically, the first capacitor C1 is constructed so as to beconsiderably larger than the second capacitor C2; for example, C1=150 nFand C2=82 pF. In this construction, for example, the second coil L2 isso constructed that a resonance frequency overall of, for example, 1 MHzis obtained for the second resonant circuit.

According to the invention, the second resonant circuit is constructedsuch that it must be operated at its natural resonance frequency f_(R2)in the ignition mode during a comparatively short resonance period orignition period of no more than a few seconds, for example 1 or 2seconds, so as to achieve the ignition of the high-pressure lamp 120.This comparatively short ignition time according to the invention hasthe advantage that the switching elements T3 and T4 in the second halfbridge 110-2 are also switched over at said resonance frequency or anodd fraction thereof during this short period only. The high-frequencyswitching operation of the switching elements T3 and T4 give rise tohigh losses therein, but these are acceptable because of the shortduration of the ignition operation.

During ignition, the switching element T1 of the first switching bridge110-1 is preferably fully switched on, whereas the second switchingelement T2 is switched off, so that the junction point between the firstcapacitor C1 and the high-pressure lamp 120 is permanently connected toa high potential. As long as the high-pressure lamp 120 has not yetignited, i.e. no lamp current can flow, this lamp is not controlled bythe first half bridge 110-1 either.

When the high-pressure lamp finally ignites, it will first be in aso-called glow discharge state. In the glow discharge state, thehigh-pressure lamp requires an operating voltage of approximately 300 V,which is substantially lower than the ignition voltage but stillcomparatively high in comparison with an operating voltage of 75 Vnecessary for normal operation, as will be described further below. Inthe glow discharge state, the voltage drop across the high-pressure lamp120 serves as a countervoltage, and it is necessary to drive asufficiently strong current through the high-pressure lamp 120 againstthis countervoltage for achieving that its electrodes heat upsufficiently, so that the high-pressure lamp 120 will enter a subsequentluminous arcing mode. The glow discharge operation damps the secondresonant circuit C2, L2 to the extent that a voltage adjusts itselfacross the second coil L2 which is just sufficient for driving therequired current through the high-pressure lamp 120. The drop in voltageacross the second coil L2 also causes its current to drop to the samerelative degree. If this current through the second coil L2 becomes tooweak for providing a sufficiently strong current to the high-pressurelamp 120, this drop may be counteracted by a decrease in the operatingfrequency of the second resonant circuit. Alternatively to a drop in theoperating frequency of the second resonant circuit, a switch may be madeto normal operation as described further below, because here it ispossible to generate at most the operational DC voltage V_(DC),approximately 400 V, for the high-pressure lamp 120. It should be noted,however, that the operating voltage V_(DC) of the circuit is notidentical to the average operating voltage of the high-pressure lamp 120in normal operation, which lies at approximately 75 V, as was notedabove.

If the ignition voltage for the high-pressure lamp 120 is not generatedby the second resonant circuit C2, L2 being operated at its naturalresonance frequency f_(R2), but instead at an odd fraction thereof, theadvantage will arise that the lower switching frequency reduces theswitching losses in the switching elements T3 and T4 in the second halfbridge 110-2 as compared with the pure resonant operation at f_(R2),while at the same time the glow discharge current through thehigh-pressure lamp 120 is increased in comparison with the resonantoperation by the factor of the fraction; i.e. in comparison with anoperation at the actual resonance frequency, the glow discharge currentis greater by a factor 3 in the case of excitation by merely ⅓of theresonance frequency.

2. Run-Up Mode After the glow discharge, the high-pressure lamp 120enters a luminous arcing mode in which it initially has a very lowburning voltage of approximately 15 V. A current flows already throughthe high-pressure lamp 120 both in the glow discharge and in theluminous arcing mode, so that the current control described above withreference to FIG. 1 is operational already in principle. The very lowoperating voltage which establishes itself in the luminous arcing mode,however, leads to a drop in the switching frequencies of the switchingelements T1 and T2 in the first half bridge 110-1. This may proceed tothe point that at the start of lamp operation, i.e. in the luminousarcing mode, the switching frequency may drop to below the resonancefrequency of the first resonant circuit comprising the first coil L1 andthe first capacitor C1 . This will then have the result that a stablecurrent control is no longer possible. It was found, however, that auseful operation of the current control is nevertheless made possible ifthe minimum switch-on duration of the switching elements T1, T2 islimited in downward direction. This does give rise to an irregularswitching pattern for the switching elements T1 and T2, as is shown inFIG. 3, with strong noise components in the lamp current, but theaverage lamp current remains continuously controllable. The irregularswitching pattern manifests itself in the irregular time durations ofthe rising and falling edges in the current through the first coil L1,because the first switching element T1 is switched on and the secondswitching element T2 is switched off during each rising edge, whereas inthe opposite case, i.e. during the falling edges, the first switchingelement T1 is switched off and the second switching element T2 isswitched on.

3. Normal Operational Mode After the run-up phase, the high-pressurelamp 120 enters the normal operational mode. In this normal operationalmode, the high-pressure lamp is supplied with a low-frequencyalternating current whose basic frequency is given by the switchingfrequency of the switching elements T3 and T4 of the second half bridge110-2.

FIG. 4 shows an example of a positive half wave of the pulsatoryalternating current through the high-pressure lamp 120. The switchingelement T4 is switched on and the switching element T3 is switched offduring the positive half wave. While the switching elements T3 and T4 ofthe second half bridge 110-2 are switched on and off alternately inaccordance with the desired basic frequency of the lamp current, theduty cycles of the switching elements T1 and T2 of the first half bridge110-1 are controlled by the current control described above withreference to FIG. 1 such that an average DC current level adjusts itselfin the first coil L1, which level corresponds to the desired currentthrough the high-pressure lamp 120.

It is apparent from FIG. 4 that the switching frequencies of theswitching elements T1 and T2 are substantially higher than the switchingfrequencies of the switching elements T3 and T4. More precisely, theswitching element T4 remains switched on throughout the time periodcovered by FIG. 4, and the switching element T3 remains switched offthroughout this entire period, whereas the switching element T1 isswitched on during each rising edge of the current through the firstcoil L1 and is switched off for the duration of the falling edges. Bycontrast, the switching element T2 is switched off during the risingedges of the current through the first coil L1 and switched on for theduration of the falling edges.

The high switching frequencies, in particular of the switching elementsin the first half bridge 110-1, may give rise to high losses therein.These losses may be substantially reduced by a so-termed voltagelessswitching of the switching elements T1, T2. This voltageless switchingmay be achieved in that each switching element T1 and T2 is given arespective parallel capacitor C4 and C3, and in that the high-frequencyalternating current through the first coil L1 passes the zero line bothin downward and in upward direction in each switching cycle. To realizethe latter, the first switching element T1 is first switched on and thesecond switching element T2 of the first half bridge 110-1 is switchedoff. This switching configuration achieves that the current through thefirst coil L1 rises to a high positive value. Once this current hasreached a desired threshold value, the switching states of the switchingelements T1 and T2 are changed over after a given delay time realized bythe delay circuit 150, so that then the switching element T1 is switchedoff. The current, which first further flows from the half bridge 110-1as in the first switching configuration, then starts to re-charge thecapacitors C3 and C4. More accurately, the re-charging takes place suchthat the voltage across the capacitor C3, and thus at the output 112-1of the first half bridge 110-1, drops, whereas the voltage across thecapacitor C4 rises. When the voltage at the output 112-1 has reached thevalue “0”, a diode present in the switching element T2, if the latter isconstructed as a MOSFET transistor, becomes conductive, and the currentin the first coil L1 starts to decrease. Now the second switchingelement T2 can be switched on without losses.

The typical operating voltage of the high-pressure lamp 120 in thenormal operational mode, i.e. of approximately 75 V, is now presentacross the capacitor C1. This voltage causes the current in the firstcoil L1 to decrease further until it finally drops below the zero line,as is shown in FIG. 4. Now the second switching element T2 of the firsthalf bridge 110-1 can be switched off without losses. The currentthrough the high-pressure lamp 120 and through the first coil L1 flowsinto the first half bridge 110-1 again starting from this moment andagain starts to charge the capacitors C3 and C4, so that the voltage atthe output 112-1 of the first half bridge rises again. When it finallyreaches the level of the supply voltage again, a diode in the firstswitching element T1, if this is constructed as a MOSFET transistor,becomes conductive. The cycle described above with the current risethrough the first coil L1 then starts from scratch again. A loss-freeswitching on and off of the switching elements T1 and T2 can becontinually maintained in this manner.

After the current direction has been reversed, i.e. in the negative halfwave, the switching element T4 is switched off and the switching elementT3 of the second half bridge 110-2 is switched on. The first half bridge110-1 is now controlled by the current control such that the averagecurrent flows into the first half bridge.

The capacitor C1 drains off the high-frequency component of the currentthrough the first coil L1 to the reference potential (−). At the sametime, the second coil L2 represents a barrier for any remaining portionsof high-frequency components in the lamp current.

As was noted above, the second capacitor C2 with the second coil L2forms a second resonant circuit whose resonance frequency typically is amultiple of the switching frequency of the first half bridge. In thenormal operational mode, accordingly, the second resonant circuit isnormally operated at a frequency far below its resonance frequency,which is why any remaining high-frequency current in the capacitor C2 isonly very small. If, for example, the first capacitor C1 is chosen to be150 nF and the second capacitor 82 pF, the remaining alternating currentin the second capacitor C2 amounts to no more than approximately 0.1% ofthe alternating current through the first capacitor C1.

FIG. 5 shows a further embodiment of the circuit according to theinvention. Its operation is the same as that of the embodiment shown inFIG. 1. The essential difference with the embodiment shown in FIG. 1 isthat the second capacitor C2 is not connected to the reference potential(−) but lies in parallel to the high-pressure lamp 120. This circuitarrangement has the same effect as the circuit configuration shown inFIG. 1 at least in the ignition mode or generally in the case in which ahigh-frequency current flows through the capacitor C2, because the firstcapacitor C1 represents a short-circuit to the reference potential (−)for high-frequency currents. The second capacitor C2 would then also beconnected—as in FIG. 1—between the junction point of the high-pressurelamp 120 and the second coil L1 on the one hand and the referencepotential (−) on the other hand.

It was furthermore found that a loss-free switching of the switchingelements T1 and T2 can also be realized with only one capacitor, inparticular the third capacitor C3, in particular at high-frequencycurrents. The fourth capacitor C4 would then be unnecessary.

FIG. 6 shows a third embodiment of the circuit according to theinvention. It differs from the first embodiment shown in FIG. 1exclusively in that capacitors are also provided parallel to theswitching elements of the second half bridge 110-2. In particular, afifth capacitor C5 is connected in parallel to the third switchingelement T3, and a sixth capacitor C6 in parallel to the fourth switchingelement T4. These capacitors render possible a reduction in theswitching losses of the switching elements T3 and T4, similar to theaction of the capacitors C3 and C4. They are particularly advantageousin the ignition mode, because the losses in the switching elements areparticularly high then because of the particularly high switchingfrequencies. The capacitors C5 and C6 in addition provide anadvantageous reduction in the edge steepness of the voltage at theoutput of the half bridge 110-2. This again is advantageous for thesuppression of HF interference.

The current control in the embodiments of FIGS. 5 and 6 operates in thesame manner as the one described with reference to FIG. 1 above.

1. An electronic circuit (100) for operating a high-pressure lamp (120)in an ignition mode and a normal operational mode, comprising a DC-ACconverter comprising a first and a second half bridge (110-1, 110-2)which are connected in parallel between an operating potential (+) and areference potential (−) for providing a suitable alternating current tothe high-pressure lamp (120) in the two said operating modes; and aseries arrangement comprising a first coil (L1), followed by thehigh-pressure lamp (120), again followed by a second coil (L2), whilethe connection terminal of the first coil (L1) not connected to thehigh-pressure lamp (120) is connected to the output (112-1) of the firsthalf bridge (110-1), and the connection terminal of the second coil (L2)not connected to the high-pressure lamp (120) is connected to the output(112-2) of the second half bridge (110-2), said outputs being eachformed by a central tap of a half bridge; characterized by a firstcapacitor (C1) which is connected in the path from the junction point ofthe first coil (L1) and the high-pressure lamp (120) either to thereference potential (−) or to the operating potential (+); and a secondcapacitor (C2) which is connected in the path from the junction point ofthe high-pressure lamp (120) and the second coil (L2) either to thereference potential (−) or to the operating potential (+) or in parallelto the high-pressure lamp (120).
 2. A circuit as claimed in claim 1,characterized in that a third capacitor (C3) is connected between theoutput (112-1) of the first half bridge (110-1) and either the operatingpotential (+) or the reference potential (−).
 3. A circuit as claimed inclaim 1, characterized in that a third capacitor (C3) is connectedbetween the output (112-1) of the first half bridge (110-1) and thereference potential (−), and in that a fourth capacitor (C4) isconnected between the operating potential (+) and the output (112-1) ofthe first half bridge (110-1).
 4. A circuit as claimed in claim 1,characterized in that a fifth capacitor (C5) is connected between theoutput of the second half bridge (112-2) and the operating potential(+), and/or in that a sixth capacitor (C6) is connected between thereference potential (−) and the output (112-2) of the second half bridge(110-2).
 5. A circuit as claimed in claim 1, characterized by a sensordevice (130) for generating a current-sensor signal which represents thevalue of the current through the first coil (L1); and by a comparatordevice (140) for comparing the value represented by the current-sensorsignal with a given reference current value I_(R) and for generating atleast one control signal for controlling the level of the currentthrough the first coil (L1) and through the high-pressure lamp (120) tothe given reference current value I_(R) through a suitable variation ofthe duty cycles of the switching elements (T1, T2) of the first halfbridge (110-1).
 6. A circuit as claimed in claim 5, characterized inthat the sensor device (130) is constructed as a magnetoresistivesensor.
 7. A circuit as claimed in claim 5, characterized by a delaydevice (150) for delaying the control signal for controlling theswitching elements (T1, T2) of the first half bridge (110-1) by a givendelay time with respect to the moment when it is detected that the levelexceeds the reference value I_(R) in upward or downward direction, whichdelay time is defined such that at least a desired critical dampingestablishes itself in a filter comprising the second coil (L2) and thefirst capacitor (C1), and that the current through the first coil (L1)changes its sign at least twice during a switching cycle of theswitching elements (T1, T2) of the first half bridge.
 8. A method ofoperating a high-pressure lamp (120) with a circuit as claimed in claim1, characterized in that the first coil (L1) and the first capacitor(C1) together form a filter for filtering out at least substantially theAC component from the current flowing through the high-pressure lamp(120), which filter is supplied with a voltage provided by the firsthalf bridge (110-1), whose frequency lies above the resonance frequencyf_(R1) of the filter (L1, C1); and in that the second coil (L2) and thesecond capacitor (C2) together form a resonant circuit which in theignition mode is supplied with a voltage provided by the second halfbridge (110-2), whose frequency corresponds to the resonance frequencyf_(R2) of the resonant circuit (L2, C2) or to an odd fraction thereof,so as to generate an ignition voltage necessary for igniting thehigh-pressure lamp (120).
 9. A method as claimed in claim 8,characterized in that the ignition mode is maintained for at least onesecond, and in that immediately after that a switch is made to thenormal operational mode.
 10. A method as claimed in claim 9,characterized in that after ignition of the high-pressure lamp (120) theswitching frequency of the second half bridge (110-2), and thus thefrequency of the current through the high-pressure lamp (120), isreduced.
 11. A method as claimed in claim 8, characterized in that theswitching elements (T1, T2, T3, T4) of the first half bridge (110-1)and/or the second half bridge(110-2) are operated by the principle ofvoltageless switching.